Simply because it shows you where you are, and shows you the way to wherever you want to be. The latest improvement is superb : you can locally store your maps on the smartphone. Very useful in case you do not have any internet connection when you are abroad.

Yes, in case you have lost your smartphone. You can have it make a lot of noise, which is useful in case you dropped it somewhere under a pillow, or you can have it send you it’s location, in case it is stolen.